Estimating Bed Shear Stress Distribution over Bottom of a Channel on the Moving Vessel

Shear stress is critical in the determination and estimation of bedload transport and morphological changes. Generally, the bed shear stress is estimated in fields using three-dimensional flow velocities acquired by an acoustic Doppler velocimeter and current profiler mounted on the channel bottom or anchored vessel. The temporal measurement and estimation of the bed shear stress by the conventional methods in a fixed point must be significantly close to the actual bed shear stress; however, the conventional methods may not provide sufficient information of spatial distribution of the bed shear stress over the entire domain, i.e., the measurements in several fixed points cannot guarantee that the measured values represent the entire domain of interest. Therefore, the present work proposed a framework for acquiring the spatial distributions of shear stress over a domain. Accordingly, a reach with a length and width of 5.6 km and 250 m, respectively, at the end of the Hyeongsan River in South Korea was surveyed through the moving-vessel measurement with ADCP measuring the velocities of that channel reach to estimate shear stress. To collect over a possible maximum spatial domain in a short duration, the moving-vessel technique was applied over the survey region without stopping a vessel. The measurement was conducted in a series of single zigzag transects instead of repeatedly transects over a same cross section. The spiked noises of velocity were observed in high frequencies at a single transect and so the noises were smoothed by moving average over a reasonable circular space. After the spatial interpolation, the accuracy of the spatial distributions of the bed shear stress was compared to the anchored-vessel measurement, and the reliable bed shear stress map was provided as a result.